The present invention relates generally to digitally-corrected temperature-compensated piezoelectric crystal oscillators (DTCXOs) having frequency outputs that are stable despite changes in the ambient temperature, and specifically to DTCXOs used in mobile communications service, such as in cellular phones and telephone call pagers.
Crystal oscillators tend to be the most stable of oscillators and so are frequently used in radio transmitters and receivers. The crystal oscillator derives its stability from the piezoelectric phenomenon exhibited by certain crystals. Cutting a piezoelectric crystal to a certain shape and size will determine its resonant frequency. The resonant frequency can be fine tuned by loading the crystal with a capacitance. Temperature changes will induce corresponding resonant frequency shifts in crystal oscillators. Since U.S. government regulations concerning radio transmitter carrier stability and accuracy are very demanding, thermal instability of crystal reference oscillators must be controlled. An early method of stabilizing crystal oscillators was to place the crystal in an "oven" that maintained a constant temperature. Because the temperature in the crystal oven did not vary, neither did the resonant frequency. Other methods have used varactor diodes to control frequency shifts by adjusting an analog voltage on the varactor according to ambient temperature. Crystals that have had their temperature-versus-resonant frequencies characterized can be corrected if the temperature is known. The technique also requires the load capacitance-to-resonant frequency shift to have been characterized. The prior are includes analog-to-digital conversion methods and the switching of a bank of load capacitors across a crystal.
FIG. 1 is typical example of a prior art cellular telephone transceiver, referred to by the general reference numeral 10, comprising an antenna 12, a transmit/receive (T/R) transfer switch 14, a bandpass filter 16, an RF amplifier 18, a first mixer 20, a receiver PLL synthesizer 22, a reference oscillator circuit 24, a first intermediate frequency (IF) bandpass filter 26, a first IF amplifier 28, a second mixer 30, a second local oscillator 32, a second IF bandpass 34, a second IF amplifier 36, a detector 38, a low-pass filter 40, a data/voice decoder 42 having a speaker 44 and an alphanumeric display 46, an alphanumeric keypad 48, a microphone 50, a frequency modulation (FM)/pulse modulation (PM) encoder 52, an encoder/decoder oscillator 54, a central processing unit (CPU) 56, a transmitter PLL synthesizer 58, a transmitter mixer 60, a transmitter amplifier 62, and a transmitter bandpass 64. An RF receiver is thus formed of elements 12-46, and a matching RF transmitter by elements 48-64, and also sharing 12-14. Receiver PLL synthesizer 22 provides a first local oscillator frequency for the first mixer 20 to beat with the incoming RF frequency to produce a first IF. Receiver PLL synthesizer 22 can be digitally programmed to output various frequencies, and therefore the frequencies received by transceiver 10 can be selectively tuned in. Reference oscillator 24 comprises a first DTCXO and supplies a master reference clock to PLL synthesizers 22 and 58. Because reference oscillator 24 provides the master reference for both transmit and receive, Federal Communications Commission (FCC) rules and good performance demand that the frequency be accurate and have minimal temperature drift. Second local oscillator 32 comprises a second DTCXO, and is similar to oscillator 24 (only the frequencies are different). Alternatively, a divider from reference oscillator 24 is used to provide the second local oscillator frequency (which will normally be a fixed frequency). Demodulator is provided by an FM discriminator, detector 38. The low-pass filter 40 allows only audio frequencies through to decoder 42. Decoder 42 will squelch speaker 44 until a proper calling code identification is received and recognized. Display 46 will keep a user informed of the status of transceiver 10. Voice input is picked up by microphone 50. Outgoing calling codes and operating modes are entered on keypad 48. If appropriate, encoder 52 will output to transmitter mixer 60 to beat with and modulate a transmit carrier frequency coming from PLL synthesizer 58. Just the desired products of transmitter mixer 60 are passed by transmitter bandpass 64. Transceiver 10 is capable of full duplex operation, so transmission will be simultaneous with reception, albeit at different frequencies. Encoder/decoder oscillator 54 supplies a common signal to encoder 52 and decoder 42, and is based on a third DTCXO similar to the first two. CPU 56 controls decoder 42, encoder 52 and the transmit and receive frequencies by virtue of its connects to PLL synthesizers 22 and 58.
Although FIG. 2 diagrams oscillator 24, it is also representative of the construction of oscillators 32 and 54. Oscillator 24 is comprised of a crystal oscillator unit 70, a switch bank controller 72, a temperature unit 74, and a timing controller 76. Unit 70 has a capacitor trimming bank 78, a capacitor switching bank 80, and a piezoelectric crystal (XTAL) 82. Switch bank controller 72 has two parts, a parallel digital latch 84 and a programmable read only memory (PROM) 86. Data in PROM 86 matches the characteristic temperature profile of XTAL 82 and also takes into account the effect of C.sub.1 -C.sub.n on the frequency output (f.sub.o) of XTAL 82. The temperature unit 74 has a temperature sensor 90 and a analog-to-digital converter (ADC) 88. At any particular temperature, a code will be output by ADC 88 to PROM 86 that will apply just the right combination of C.sub.1 -C.sub.n to bring f.sub.o back to nominal. (At least according to ideal XTAL 82 performance parameters.) Controller 76 has three output signals: 92, 94, and 96. Periodically, controller 76 will cause ADC 88 to begin a new conversion. (Also see FIG. 6 discussion below.) The output of ADC 88 and PROM control signal 94 will cause a particular digital correction word to be output from PROM 86. That word will be output to latch 84 and loaded by signal 92. Latch 84 controls switch bank 80, which in turn can switch in and out various capacitors in capacitor bank 78 in order to trim the resonant frequency of XTAL 82 and keep it stable despite ambient temperature variations. Oscillator 24 is therefore a digitally-corrected temperature-compensated crystal oscillator (DTCXO). Control of the oscillating frequency of the DTCXO is continuous, regardless of whether the radio is active or not.
Prior art DTCXO circuits generate spurious frequency fluctuation noise (FM) and phase fluctuations noise (PM) whenever the DTCXO circuits vary the temperature correction (for example, by switching on and off capacitors (C.sub.1 -C.sub.n in capacitor bank 78). As was mentioned above, capacitors C.sub.1 -C.sub.n are binary-weighted and are combined to produce various correction totals. As an example, capacitors C.sub.1 -C.sub.n could be weighted as: 1, 2, 4, 8, and 16 picofarads. A range of 0-31 picofarads could then be accommodated in one picofarad steps. The smallest step, one picofarad, results in a certain granularity that will cause oscillator 24 (for example) to display discrete frequency steps when the temperature compensation is hunting. These frequency and phase steps, or fluctuations will consequently be reflected by similar modulating the transmitter and receiver frequencies (which add audible noise to the transmitted and received signals). These spurious modulations can interfere not only with the voice, but data traffic too, and at both ends of the radio communication. If oscillator 54 also shows such fluctuations, the encoder and decoder processes timed by it can experience a loss in signal synchronization, which will necessitate a switch being made to an alternative, "cleaner" channel (when it really wasn't necessary).
Reducing the amount of stepping in a DTCXO frequency control can reduce the impact of any frequency and phase fluctuations generated by the temperature compensation. But the resolution of the temperature measurements and the resolution of the frequency adjustments both would have to be increased, and that would greatly increase the width of the digital control word D.sub.1 -D.sub.n. To support these increases, a higher quality temperature sensor would also be required. Very fine frequency control, such as this, requires very complex circuits and increases the cost of manufacture.